The proposed experiments are designed to determine the structures of the protein complexes at the membrane interface in G protein signaling systems. The well-characterized vertebrate rod vision transduction pathway will be used as a model. By tethering specialized probes at specific sites on the surface of the G protein transducin, fluorescence resonance energy transfer and electron paramagnetic resonance spectroscopy will be utilized to determine the relative distance to the membrane. These distances will be tested for the inactive conformation of the protein, as well as with the activated conformation, and in the presence of downstream effectors.